Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Multi-atom cluster catalysts for efficient electrocatalysis

This review presents recent developments in the synthesis, modulation and characterization of multi-atom cluster catalysts for electrochemical energy applications.

Activating Basal Surface of Palladium by Electronic Modulation via Atomically Dispersed Nitrogen Doping for High-Efficiency Hydrogen Evolution Reaction

Surface doping by atomically dispersed heteroatoms has become one of the most promising strategies for facilitating the catalytic activity of non-noble transition metals to replace platinum-based catalysts in the hydrogen evolution reaction (HER). However, the underlying mechanism for the atomically dispersed heteroatoms to modulate the electronic structure and the HER activity of a metal surface is still ambiguous. Moreover, the active catalytic region is limited by the small fraction of doped atoms, and the remaining basal surface is inactivated. Here, we demonstrate that the nitrogen doping is atomically dispersed on the palladium surface, which can achieve the near-thermoneutral hydrogen adsorption and promote the HER activity of the basal surface. The theoretical modeling reveals that the dispersed nitrogen atoms attract electrons from palladium and downdrift the d-band center for accelerating the hydrogen desorption. Our work offers understandings of atomically dispersed nitrogen doping on the surface of transition metals and paves the way for a further optimization of nonprecious-metal HER electrocatalysts.

Hydrogen release from a single water molecule on Vn+ (3 ≤ n ≤ 30)

Water and its interactions with metals are closely bound up with human life, and the reactivity of metal clusters with water is of fundamental importance for the understanding of hydrogen generation. Here a prominent hydrogen evolution reaction (HER) of single water molecule on vanadium clusters V_n⁺ (3 ≤ n ≤ 30) is observed in the reaction of cationic vanadium clusters with water at room temperature. The combined experimental and theoretical studies reveal that the wagging vibrations of a V-OH group give rise to readily formed V-O-V intermediate states on V_n⁺ (n ≥ 3) clusters and allow the terminal hydrogen to interact with an adsorbed hydrogen atom, enabling hydrogen release. The presence of three metal atoms reduces the energy barrier of the rate-determining step, giving rise to an effective production of hydrogen from single water molecules. This mechanism differs from dissociative chemisorption of multiple water molecules on aluminium cluster anions, which usually proceeds by dissociative chemisorption of at least two water molecules at multiple surface sites followed by a recombination of the adsorbed hydrogen atoms.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N₂ on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh₃N_2m-1⁺ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh₃N₇⁺ is identified with outstanding mass abundance among the Rh_nN_2m-1⁺ products, and its ground-state structure is in the form of Rh₃N(N₂)₃⁺ by capping a nitrogen atom on the top of Rh₃⁺ plane and hanging three N₂ molecules beneath the three Rh atoms respectively, giving rise to a C_3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N₂ molecules with two rhodium clusters under plasma atmosphere.

Hollow Mesoporous Carbon Sphere Loaded Ni-N4 Single-Atom: Support Structure Study for CO2 Electrocatalytic Reduction Catalyst

Single-atom catalysts have become a hot spot because of the high atom utilization efficiency and excellent activity. However, the effect of the support structure in the single-atom catalyst is often unnoticed in the catalytic process. Herein, a series of carbon spheres supported Ni-N₄ single-atom catalysts with different support structures are successfully synthesized by the fine adjustment of synthetic conditions. The hollow mesoporous carbon spheres supported Ni-N₄ catalyst (Ni/HMCS-3-800) exhibits superior catalytic activity toward the electrocatalytic CO₂ reduction reaction (CO₂ RR). The Faradaic efficiency toward CO is high to 95% at the potential range from -0.7 to -1.1 V versus reversible hydrogen electrode and the turnover frequency value is high up to 15 608 h^-1 . More importantly, the effect of the geometrical structures of carbon support on the CO₂ RR performance is studied intensively. The shell thickness and compactness of carbon spheres regulate the chemical environment of the doped-N species in the carbon skeleton effectively and promote CO₂ molecule activation. Additionally, the optimized mesopore size is beneficial to improve diffusion and overflow of the substance, which enhances the CO₂ adsorption capacity greatly. This work provides a new consideration for promoting the catalytic performance of single-atom catalysts.

Design of a graphene nitrene two-dimensional catalyst providing a well-defined site accommodating up to three metals, with application to N2 reduction electrocatalysis

We consider here a novel two-dimensional catalyst M₃-C₂N that we apply to electrocatalytic N₂ reduction. Using the PBE-D3 flavor of density functional theory (DFT), we studied 12 choices for the M₃ metal clusters to find two cases: homonuclear Rh₃-C₂N and heteronuclear Co₂Mo-C₂N that are particularly promising for electrocatalytic reduction of N₂.

Reactivity of Cobalt Clusters Con±/0 with Ammonia: Co3+ Cluster Catalysis for NH3 Dehydrogenation

A customized reflection time-of-flight (Re-TOF) mass spectrometer combined with a 177 nm deep-ultraviolet laser has enabled us to observe well-resolved cobalt clusters Co_n^±/0 and perform a comprehensive study of their reactivity with ammonia (NH₃). The anions Co_n^- are found to be inert, the neutrals allow the adsorption of multiple NH₃ molecules, while the cationic Co_n⁺ clusters readily react with NH₃ giving rise to dehydrogenation. However, incidental dehydrogenation of NH₃ on Co_n⁺ is only observed for n ≥ 3. The dramatic charge- and size-dependent reactivities of Co_n^±/0 clusters with NH₃ are studied by the density functional theory (DFT)-calculation results of energetics, density of states, orbital interactions, and reaction dynamics. We illustrate the dehydrogenation from two NH₃ molecules, where a significantly reduced transition-state energy barrier is found pertaining to the dimolecular co-catalysis effect. The reactivity of Co₃⁺ with NH₃ is illustrated showing effective catalysis for N-H dissociation to produce hydrogen applicable for designing ammonia fuel cells.